1. Field of the Invention
The present invention relates to a dielectric ceramic and a fabrication method therefor, and a monolithic ceramic capacitor incorporating the dielectric ceramic. In particular, it relates to an improvement that effectively achieves thicknessreduction of dielectric ceramic layers of the monolithic ceramic capacitor.
2. Description of the Related Art
Monolithic ceramic capacitors are typically fabricated as follows.
First, a powder mainly containing a dielectric ceramic, for instance, BaTiO3, is pulverized into primary particles as much as possible using a ball mill or the like and the pulverized powder is dispersed in a matrix to prepare a ceramic slurry. The ceramic slurry is shaped into ceramic green sheets. A conductive material, which will form internal electrodes, is applied on the surfaces of particular ceramic green sheets to form predetermined patterns on the surfaces.
A plurality of ceramic green sheets with the conductive material thereon are stacked and bonded through thermal compression to form a green composite.
The green composite is baked to form a sintered compact. The sintered compact has the internal electrodes composed of the above-described conductive material inside.
Subsequently, external electrodes that electrically connect with particular internal electrodes are formed on the outer surfaces of the compact. The external electrodes are formed, for example, by applying a conductive paste containing a glass frit and a conductive metal powder onto the outer surfaces of the compact and baking the applied paste. Thus, a monolithic ceramic capacitor is made.
Conventionally, palladium or a palladium-silver alloy has been used to make the internal electrodes. Recently, a relatively inexpensive base metal, such as nickel or copper, has been frequently employed to reduce the manufacturing cost of monolithic ceramic capacitors. In making monolithic ceramic capacitors having internal electrodes composed of a base metal, the baking step must be performed in a neutral or reducing atmosphere to prevent oxidation of the base metal. Thus, the dielectric ceramic used in the monolithic ceramic capacitor must be resistant to reduction.
In making a monolithic ceramic capacitor having a capacitance-temperature characteristics that comply with the B characteristics of Japanese Industrial Standards (JIS), a reduction-resistant dielectric ceramic containing BaTiO3 as the main component, an oxide of a rare earth element, an acceptor element, such as Mn, Fe, Ni, or Cu, and a sintering aid has been employed.
For example, Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, 5-9068, and 9-270366 teach compositions of dielectric ceramics that exhibit a high dielectric constant with relatively small change with temperature and longer hightemperatureload life.
From the viewpoint of the structure and texture of the dielectric ceramic, Japanese Unexamined Patent Application Publication Nos. 6-5460, 2001-220224, and 2001-230149 teach dielectric ceramics having a so-called core-shell structure.
Japanese Unexamined Patent Application Publication No. 2001-313225 teaches a core-shell structure dielectric ceramic having the core partly exposed from the shell.
Recent advancements in electronics has accelerated the miniaturization of electronic components. Monolithic ceramic capacitors are becoming ever smaller while maintaining large capacitance. One effective measure for achieving both size-reduction and large capacitance is to reduce the thickness of dielectric ceramic layers of the capacitor. The thickness of the dielectric ceramic layers has been reduced to about 2 xcexcm or less in manufactured capacitors and about 1 xcexcm in experimental capacitors.
In order for the electric circuit to stably operate despite changes in temperature, the capacitance of the capacitor used in the circuit must also be a stable relative to temperature changes.
Thus, a monolithic ceramic capacitor having a capacitance that does not change largely relative to temperature changes and that exhibits superior electrical insulation and high reliability even when the thickness of the dielectric ceramic layers is reduced has been strongly desired.
The dielectric ceramics disclosed in the aforementioned Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068 satisfy the X7R characteristic of Electronic Industries Alliance (EIA) standards and exhibit superior electrical insulation. However, the capacitance-temperature characteristic and the reliability of these ceramics have not been sufficient to meet the demand of the market, especially when the thickness of the dielectric ceramic layers are reduced to about 5 xcexcm or less or, in particular, about 3 xcexcm or less. The dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 9-270366 also has a problem of degraded capacitance-temperature characteristic and reliability as the thickness of the dielectric ceramic layers is reduced.
The core-shell structure dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication Nos. 6-5460, 2001-220224, and 2001-230149 have cores surrounded by shells. Since the thermal expansion coefficient of the core is different from that of the shell, an internal pressure, such as hydrostatic pressure, is applied on the core from the shell in the course of cooling after sintering. In general, the Curie point becomes lower when a hydrostatic pressure is applied on a ferromagnetic material such as BaTiO3. Moreover, the Curie point of the dielectric ceramic mainly composed of BaTiO3 is around 120xc2x0 C. to minimize temperature dependency of the dielectric constant. Since the cores of the dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 6-5460, 2001-220224, and 2001-230149 are mainly composed of BaTiO3, the monolithic ceramic capacitors that employ this type of dielectric ceramic suffer from degradation in capacitance-temperature characteristics particularly when the dielectric ceramic layers are thin. Moreover, the reliability is also poor.
The structure of the dielectric ceramic described in Japanese Unexamined Patent Application Publication No. 2001-313225 is made by controlling the sintering temperature. Electrical characteristics of such dielectric ceramic vary easily as a result, and the capacitance-temperature characteristics and the reliability cannot be stably achieved when the thickness of the dielectric ceramic layers is small.
Accordingly, the capacitance-temperature characteristics of the monolithic ceramic capacitors will be significantly degraded if the thickness of the dielectric ceramic layers is reduced to meet the demand for miniaturization and higher capacitance without changing the AC signal level. This is because the electric intensity applied to each dielectric ceramic layer will be increased. The reliability will also be degraded if the thickness of the dielectric ceramic layers is reduced without changing the DC rated voltage for the same reason.
Monolithic ceramic capacitors that can exhibit the same temperature dependency of the dielectric constant and superior reliability with thinner dielectric ceramic layers are desired.
An object of the present invention is to provide a method for making a dielectric ceramic and a monolithic ceramic capacitor composed of the dielectric ceramic that can overcome the aforementioned problems of the prior art.
To achieve this object, a first aspect of the present invention provides a dielectric ceramic including ABO3 as the main component and a rare earth element, wherein A represents barium which may be partly replaced with at least one of calcium and strontium, and B represents titanium which may be partly replaced by at least one selected from zirconium and hafnium.
At least 70% of crystal grains of the dielectric ceramic have a cross-section in which a first region containing dissolved rare earth element occupies 5 to 70% of the area of the cross section and a second region free of the dissolved rare earth element occupies 10 to 80% of the periphery of the cross-section.
Preferably, the average concentration of the rare earth element inside the crystal grains is about half or less than half the average concentration of the rare earth element at the boundaries in the dielectric ceramic consisting of the crystal grains and grain boundaries.
The dielectric ceramic may further contain at least one acceptor element selected from manganese, nickel, iron, copper, magnesium, aluminum, chromium and vanadium.
The dielectric ceramic according may further contain a sintering aid containing at least one of silicon, boron and lithium.
A second aspect of the present invention provides a method for making the dielectric ceramic having the steps of preparing ABO3 aggregates each composed of a plurality of primary particles, the ABO3 aggregates being prepared by synthesizing ABO3, wherein A represents barium which may be partly replaced with at least one of calcium and strontium, and B represents titanium which may be partly replaced with at least one selected from zirconium and hafnium; preparing a compound of a rare earth element; blending the ABO3 aggregates with the compound and calcining the resulting mixture so that the rare earth element diffuses and dissolves in surface regions of the ABO3 aggregates to prepare a calcined powder; and sintering the calcined powder.
Preferably, the step of preparing the ABO3 aggregates further includes a substep of pulverizing the ABO3 aggregates so that most of the aggregates are composed of about four to nineteen primary particles.
Preferably, the method further includes a step of pulverizing the calcined powder into primary particles prior to the sintering step.
A third aspect of the present invention provides a monolithic ceramic capacitor that includes a composite composed of a plurality of dielectric ceramic layers and internal electrodes disposed along the interfaces of particular dielectric ceramic layers; and external electrodes disposed on outer surfaces of the composite and electrically connected to particular internal electrodes. The dielectric ceramic layers are made of the above-described dielectric ceramic.
A monolithic ceramic capacitor having dielectric ceramic layers composed of the dielectric ceramic of the present invention can exhibit good capacitance-temperature characteristics and high reliability. The thickness of the dielectric ceramic layers can be reduced to achieved size-reduction and higher capacitance. In particular, the thickness of the dielectric ceramic layers can be reduced to approximately about 0.5 xcexcm without causing problems.
With the dielectric ceramic having the average concentration of the rare earth element inside the crystal grains half or less than half the average concentration of the rare earth element at the boundaries, reliability can be further improved.
According to the method of the present invention, the ABO3 aggregates are mixed with a rare earth element compound and calcined so as to allow the rare earth element to diffuse and dissolved in the surface regions of the ABO3 aggregates to prepare the calcined powder, and the calcined powder is sintered. Thus, the dielectric ceramic that satisfied the aforementioned requirements can be securely made.
Moreover, by pulverizing the ABO3 aggregates with dissolved rare earth element into primary particles prior to sintering, the thickness reduction of the dielectric ceramic can be further promoted.